The present invention claims priority and contains subject matter related to Japanese Patent Application No. 10-166939, filed with the Japanese Patent Office on Jun. 15, 1998, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a coplanar stripline for use in high-frequency transmission, particularly to the stripline provided with a corrugated structure, having an improved transmission capability.
2. Description of the Background
A coplanar stripline is known as a transmission line for high frequency electromagnetic waves such as those in microwave and millimeter wave regions, which has been incorporated in a monolithic microwave integrated circuit (MMIC) and microwave integrated circuit (MIC). The stripline is a coplanar type of the distributed constant circuit, which is constituted of a pair of strip conductor lines.
FIG. 4 illustrates electromagnetic field lines, or electric field and magnetic field component lines for the electromagnetic wave during the propagation. Referring to FIG. 4, a coplanar line 104 is composed of a pair of strip conductors 102,103, each formed on a dielectric substrate 101 parallel to each other and separated by a predetermined distance. The construction of conductors in the coplanar striplines may thus be considered complimentary to that of the prior art coplanar waveguide.
The field lines includes the components of the electric field 401 and magnetic field 402, of the electromagnetic wave, respectively. As shown in FIG. 4, it is known the components of the electric field 401 and magnetic field are located primarily in the area between, and surrounding, the strip conductors 102,103, respectively.
The coplanar striplines have not been used often for several reasons such as a transmission loss larger than that of either micro striplines or coplanar waveguides and a difficulty caused by a complicated signal transfer to micro-strip lines.
Technological advances in the area of MMIC packaging are rapidly reducing the size of high frequency electronic assemblies. The distance of signal transmission is therefore reduced in the miniaturized circuits, and the transmission loss described above is becoming less significant.
In addition, along with the recent trend of the main stream of the high-frequency circuits toward uniplanar circuits from the previous circuits including microstrip lines, coplanar striplines and slot lines have been attracting much attention recently. Being able to be constructed with finite breadth of conductors, the coplanar striplines, in particular, are considered more advantageous, in principle, for miniaturization in contrast to the coplanar waveguides and slotlines, for which conductor strips of semi-infinite breadth have to be generally assumed.
Although the coplanar striplines are considered advantageous for the miniaturization because of finite breadth of conductors, as described above, the present inventors have realized shortcomings thereof, in that a plurality of the known coplanar striplines can not be disposed in close vicinity to each other or densely such as in an integrated circuit, because of the disturbance by crosstalk between the striplines. The present finding is based on the fact that the distribution of the electric field 401 and magnetic field 402, of the electromagnetic wave of the present propagation mode, is somewhat different from the distribution shown previously in FIG. 4.
As described earlier, the electric field components 401 have been considered to be located primarily in the area between the strip conductors 102,103, as shown in FIG. 4.
FIG. 5 is a perspective view of a coplanar stripline accompanied by an electric field 401 (represented by solid lines) and a magnetic field 402 (represented by dashed lines.
Referring now to FIG. 5, the difference mentioned above will be detailed hereinbelow, and is resolved by the present inventors. In addition to the electric field components 401 located primarily in the area between the strip conductors 102,103 (FIG. 4), the present inventors have found an additional, concentrated distribution of the electric field component exists in the outside portion of the coplanar striplines 104.
By the term xe2x80x9coutside portionxe2x80x9d, it is meant a side edge portion of each of the strip conductors 102,103, which is positioned longitudinally and not facing to the other confronting strip conductor of the coplanar stripline. These side portions are shown in FIG. 5, as the left-hand side of the strip 102 and the right-hand side of the strip 103 both located on dielectric substrate 101.
These side edge portions are hereinafter referred to as the outside portions, while the inside portions are those facing to the side edge of the other confronting strip conductor.
Referring now to FIG. 5, when an intense electric field 501 is applied to the outside portions of the strip conductors 102,103, this gives rise to a large crosstalk between the neighboring stripline which is disposed in close vicinity to each other, to thereby cause a problem, in that a plurality of planar striplines can not be disposed close together. In such a circumstance, therefore, the aforementioned advantage for the coplanar stripline can not be fully utilized, for which a finite breadth for the strip conductor can be assumed, and therefore it cannot be any longer considered advantageous for achieving the miniaturization of the circuits.
The electric field distribution in FIG. 5 is originated from the distribution of electric current flows illustrated in FIG. 6, in which a plurality of the arrows 601,602 designate the current flows including their direction. The current flows 601 are known previously to correspond to the electric field 401 of FIG. 4 which is located in the in-between portions of the strip conductors 102,103 both located on dielectric substrate 101, while other current flows 602 correspond to the electric field distribution applied to the outside portions of the strip conductors 102,103.
As clearly seen from the arrows for illustrating the current flows shown in FIG. 6, there exist the current flows along the length of the strip conductor not only in the inside portions, but also in the outside portions, of the strip conductors 102,103. The electric field 501 shown in FIG. 5 is thus induced by the current flows 602, thereby causing cross-talks between the neighboring striplines.
It is therefore an object of the present invention to provide a coplanar stripline, which overcomes the above-noted difficulties.
It is another object to provide coplanar striplines which is provided with a corrugated structure including a plurality of slits, having an improved transmission capability.
To achieve the foregoing and other objects, and to overcome the shortcomings discussed above, a coplanar stripline is provided for transmitting electrical data signals, including a dielectric substrate and a plurality of strip conductors formed on the dielectric substrate, substantially parallel to one another and separated by a predetermined distance. At least on one side of the strip conductors, a corrugated structure is provided, including a plurality of slits formed on the side of the strip conductor, so that the length direction thereof is substantially perpendicular to the direction of the electromagnetic wave transmitted through the coplanar stripline.
According to one aspect disclosed herein, a coplanar stripline is provided for transmitting electrical data signals, including a dielectric substrate, and first and second strip conductors formed on the dielectric substrate, substantially parallel to each other and separated by a predetermined distance. At least on one side of the strip conductors, a corrugated structure is provided, including a plurality of slits formed periodically on the side of the strip conductors, located in the outside portion of the stripline and not facing to the other confronting strip conductor, so that the length direction of the slit is substantially perpendicular to the direction of the electromagnetic wave transmitted through the coplanar stripline.
According to another aspect disclosed herein, the corrugated structure is formed so as to satisfy a first condition expressed by the equation
CL greater than xcex/10,
where CL is the depth of the slit and xcex is the effective wavelength of the electromagnetic wave which is currently transmitted through the coplanar stripline.
According to still another aspect disclosed herein, the corrugated structure is formed so as to satisfy a second condition expressed by the equations
CPxe2x88x92CW less than xcex/4 and CW less than xcex/4,
where CP and CW are the period and width of the slit, respectively.
According to another aspect disclosed herein, the corrugated structure is formed so as satisfy a third condition expressed by the equation
CL greater than CW.
With the coplanar stripline disclosed herein, having the corrugated structure formed periodically on the outside portions of the strip conductor, the current flows along the length of the conductors in the outer edge portions of the strip conductors and the electric field induced in those portions can be suppressed, to thereby reduce the crosstalk between the coplanar striplines. A denser circuit alignment with a plurality of striplines therefore becomes feasible.